I'm reading the comments on steam and I don't think you're handling your PR effectively. Comments like:
Its fine. You won't be missed.
...is just not something you should say when reacting to criticism IMHO. As an ambassador for your game, you shouldn't stoop to the same level as the criticism you are facing. I realize you're annoyed by some of the comments there but you have nothing to win by replying with hostility. It gives the impression that you can't handle criticism. For a lot of people on steam, they're just waiting for a dev to get into a verbal fight and you just bit that bait pretty hard. You have nothing to win here by reacting that way.
Some people will start arguing with them, other users. State your position calmly by quoting official sources like you did. To be fair, I think some of them provided some reasonable input and you seemed to admit so yourself.
I'm saying this with your best interest at heart. I know because I sometimes react impulsively too and regret it after. Writing something out of anger rarely turns out for the best. If anything, people will see they can get a rise out of you and other people will join in for some mayhem and it all goes downhill from there.
And as expected, Wilco responded in kind, spurred by your reaction. I mean, the guy is clearly not stupid:
"There are two categories in Steam Greenlight: One for mostly-finished games seeking distribution via Steam and one for early builds and concepts that are simply seeking feedback from the community. You can choose the right category for your title when you post."
What he quoted is not false. Your game would make more sense as Early Access than as Greenlight. I actually agree with him on that specific point.
This is also a valid argument, especially the point in bold:
I'm not sure I agree with you that the FAQ is either contradictory or unclear. It seems pretty straightforward. While you could say that 'mostly finished' is somewhat vague, it's definitely not in the realms of 'two-and-a-half-years away' vague.
I'm not trying to assign 'fault' (and I don't really care if you want to blame your artist or not) but I am interested to know that since you're now aware of the requirement, what your actions are going forward as it's those that define you.
I mean, I agree with that too. It's seems pretty clear cut that there was a mistake here when your artist submitted the game and didn't read the FAQ thoroughly. Instead of blaming the community as being ferocious, I would certainly try to steer the ship around and admit the mistake, it'll be a lot more beneficial for the game in the long run that way (which I'm sure it's what you want).
Also, saying it's the artist's fault openly like that is also a faux-pas, you're putting him on the spot and this gives the impression that there's dysfunctional communication in your team and lack of cohesion. I know this would piss me off unless it was agreed beforehand.
I hope you'll roll with the punches on this one. Good luck!